Generally, an automatic transmission controls hydraulic pressure by operating at least one solenoid valve according to a driving state, such as a vehicle speed or a throttle opening, in order to shift to a target shift speed. In a case that shifting to the target shift speed is performed, an automatic transmission has an off-going element that changes from an engaged state to a released state and an on-coming element that changes from a released state to an engaged state. Releasing an off-going element and engaging an on-coming element is realized by controlling a hydraulic pressure supplied to respective elements. In addition, control of the hydraulic pressure supplied to the off-going and on-coming elements is performed by operating a control duty and controlling the solenoid valve according to the control duty.
Meanwhile, a vehicle frequently changes its state from power-on to power-off or from power-off to power-on as a consequence of electric loads, such as an air conditioner, when a downshift from third gear to second gear is performed before the vehicle stops. Accordingly, a shifting process may be unstable and shift shock may occur.
Generally, a downshift from third to second before stopping is performed in a power-off state. In this case, since an angular velocity of an engine is smaller than an angular velocity of a turbine, shift shock may not occur. However, in a case that an air conditioner is operated, the angular velocity of the engine is larger than the angular velocity of the turbine as a consequence of operation of a relay, and accordingly, the downshift before stopping is performed in a power-on state and shift shock may occur.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.